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Project Summary

The isotope ratios of oxygen and silicon are a sensitive monitor of sedimentary and hydrothermal processes for deposition of banded iron formation. Our focus on banded iron formations reflects the importance these unusual units have in biogeochemical cycling in the early Earth. In particular, we are examining the deposition of microlaminated sections of the Dales Gorge member of the Brockman Iron Formation from the Hamersley basin, which have alternatively been interpreted to represent annual varves in chemical precipitates from seawater, or variations in a sub-surface hydrothermal system. These conflicting models have relevance for interpreting the role of microbial life in precipitation of the Fe-oxides. In addition, δ18O and δ³⁴S values from coexisting minerals can be used to estimate the temperature of the Archean ocean, or of the hydrothermal system. δ⁷Li and δ18O values in zircons allow these tests to be applied to magmas that may have assimilated sedimentary materials, and, in the case of Jack Hills samples from SW Australia, provide a record of the earliest Earth (4.4 to 4.0 Ga), before the formation of all known rocks.